Existing PLC with integrated web server send two types of information to a remote location: data and instructions for presentation of this data at the remote location. The data may change during every scan cycle of the PLC, while the instructions are usually changed only if the PLC project is updated. Therefore, the instructions are updated much less frequently than the data changes. However, the PLC sends both data and instructions in response to every data request from the remote location. This results in significant duplication of communication load.
With development of remote interaction features, complexity of data presentation and, therefore, the amount of instructions for data presentation will grow. It will be critical soon to minimize the communication load and required storage space in PLC for presentation instructions.
Additionally, operation with PLC remotely through the Internet requires special security means to protect PLC control functions against non-authorized access. But also intranet scenarios require better security.
Also, different users at remote locations may want to have different customized web pages for interaction with the same PLC. It may be also desirable to provide users having different access right with different designs of web pages.
U.S. Pat. No. 6,640,140 discloses a PLC that contains web pages comprising data and associated instructions retrievable and executable by the remotely located computer. Thus, the PLC contains not only data retrievable by the remotely located computer, but rather in response to a request for data the web server, which is hosted by the programmable logic controller device, provides one or more web pages in which the requested data are displayed. U.S. Pat. No. 6,640,140 further discloses a method wherein the step of storing web pages comprising data and associated instructions retrievable and executable by the remotely located computer is performed according to instructions hosted by the same programmable logic controller device. Furthermore, the step of responding to a request for data from the remotely located computer by providing one or more of the web pages used to display the requested data is performed according to instructions hosted by the same programmable logic controller device.